1. Field of the Invention
The present invention relates to a polymer having a fluorine-containing end group which is obtained by radical polymerization of a monomer capable of radical polymerization in the presence of a fluorinated thiol, and also to a process for producing said polymer.
2. Description of the Prior Art
It is known well that fluorocarbons provide characteristic surfaces which are hard to get wet and to become dirty and are slippery, on account of the extremely low surface energy they have. They are also superior in heat resistance and chemical resistance. Because of these characteristic properties, fluorocarbons have recently come into use for surface finish, and the surface finish with fluorocarbons is finding use in many application areas.
Fluorocarbon surfactants are superior in interfacial properties to hydrocarbon surfactants. They draw public attention because they produce a good effect even when used in small quantities. However, since fluorocarbon compounds are generally expensive, it is expected that they will find use more in the area where their characteristic surface properties are of importance, than in the area where their bulk properties are utilized. Typical examples of their use include the soil-resistant finishing that imparts water repellency and oil repellency to textiles and film.
For example, Japanese Patent Laid-open No. 139679/1979 discloses a process for producing surface-treated polyvinyl alcohol film. According to this process, a dispersion or solution of fluoroplastic such as tetrafluoroethylene is applied, followed by drying, to the surface of polyvinyl alcohol film. The surface treatment of a material with a fluorocarbon compound, as in the above case, has a disadvantage that sufficient adhesion is not achieved between the substrate material and the fluorocarbon compound coated because the characteristic surface properties of the fluorocarbon compound interfere with adhesion, in other words, the superior surface properties of the fluorocarbon compound do not last for a long time of period. Thus there is a demand for a new technology that overcomes this disadvantage.
One way of improving the durability is to chemically bond the fluorine-containing compound to the surface of a polymeric substrate through graft polymerization which is usually achieved by irradiation or electrical discharge. Graft polymerization by irradiation is not generally used because irradiation severely deteriorates the polymeric substrate. Graft polymerization by electrical discharge is in limited use as disclosed in Japanese Patent Laid-open No. 56671/1979 which claims a fluorinated polymeric substrate having the surface layer formed by graft polymerization of a fluorine-containing unsaturated monomer by electrical discharge.
Graft polymerization by electrical discharge is achieved by any one of the following ways, which all require complex processes and operation.
(1) The surface of the substrate is activated by electrical discharge and then exposed to a vapor of fluorine-containing monomer. (This is suitable for a fluorine-containing monomer of high vapor pressure.) PA0 (2) The surface of the substrate is activated by electrical discharge and then brought into contact with a solution of fluorine-containing monomer. (This is suitable for a fluorine-containing monomer of low vapor pressure.) PA0 (3) The surface of the substrate is activated by electrical discharge and exposed to oxygen or an oxygen-containing gas and finally brought into contact with a heated solution of fluorine-containing monomer. In addition, graft polymerization by electrical discharge does not form a uniform layer on a polymeric substrate having an intricate, irregular surface. PA0 (a) Example of synthesis of fluorinated thiol: PA0 (b) Production of polymer having a fluorinated end group:
In order to overcome the above-mentioned disadvantages, the present inventors carried out a series of researches on the process for modifying the plastics surface with a fluorocarbon compound. As the result, it was found that an improved surface is obtained by the aid of a polymer having a fluorine-containing end group which is produced by polymerizing monomers capable of radical polymerization in the presence of a fluorinated thiol.
The present inventors also investigated the method for improving the surface of polyvinyl ester and polyvinyl alcohol obtained by hydrolyzing said polyvinyl ester. This investigation led to the following findings. When a vinyl ester such as vinyl acetate undergoes radical polymerization in the presence of a fluorine-containing thiol, there is obtained a polyvinyl ester such as polyvinyl acetate having a fluorine-containing end group. This polymer, upon saponification, provides polyvinyl alcohol having a fluorine-containing end group. These polymers thus obtained have the surface characteristic of fluorocarbon compound. The present invention is based on these findings.